Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly/idempotent/non-destructive, and the description adds substantial context beyond them: rates are computed over messages SENT not the audience, rows_expired age-out behavior, and that this read is NOT gated by the Send Message module so a 403 has another cause. This is exactly the kind of behavioral detail annotations cannot carry.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the report's contents before the caveats, and each sentence (denominator warning, rows_expired, 403 troubleshooting) carries distinct operational value. It is dense with three separate caveats, but none is filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description compensates by enumerating the report's sections and explaining the two subtle return semantics (denominator choice and rows_expired degradation) plus the 403 behavior. An agent has everything needed to call and interpret it correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Only one parameter (campaign_token), whose UUID v4 format is fully documented in the schema (100% coverage). The description adds no syntax or format guidance beyond what the schema already states, so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Delivery report of one campaign') and then enumerates exactly what the report contains (counters, rates, messages by status, top error codes, sends per hour, most-clicked links). This clearly distinguishes it from siblings like fetch_campaign_recipients, which gives per-recipient detail rather than aggregate counters.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives strong operational guidance: it warns the agent to state the denominator when reporting percentages, explains that rows_expired=true means the underlying detail is gone, and instructs that a 403 here should not be blamed on the Send Message module. It stops short of explicitly naming an alternative tool for a different use case, so it's clear context without full alternative routing.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.